Magnetic lens for electron microscopes



J ly 30, 1 40- B. VON BORRIES ET AL 956 MAGNETIC LENS FOR ELECTRON MICROSCOPES Filed Aug. 6, 1938 030% Van M Ma.

Patented July 30, 1940 UNITED STATES PATENT OFFlCE MAGNETIC LENS FOR ELECTRON MICROSCOPES Germany Application August 6, 1938, Serial No. 223,458 In Germany August 16, 1937 4 Claims.

The invention relates to a magnetic lens such as may, for instance, be employed in electron microscopes of the type disclosed for instance in our copending application, Serial No. 225,278.

To attain small focal lengths necessary for this type of microscope, magnet pole systems must be used in such lenses whose pole pieces are so arranged as to be spaced from one another about 1 mm. or less. In the case of a symmetrical design of the pole pieces the object can therefore hardly be placed suficiently close to the magnetic lens. Consequently, it has been proposed to use asymmetric pole pieces in suchv a manner that one pole piece represents a rather obtuse-angled cone and the other pole piece a rather acute-angled cone. Thus it becomes possible to place the object at the side of the obtuse-angled pole piece suificiently close to the lens.

According to the invention an asymmetric magnet pole system of the above-described character is designed so that the magnetic saturation occurs at least at the ends of both pole pieces practically at the same time. Thus it is possible to obtain in the simplest manner a magnetic lens of a particularly short focal length.

The invention is illustrated in the accompanying drawing, in Which Figs. 1 and 2 represent semi-diagrammatically central sectional elevations of two forms in which so as examples the invention may be reduced to practice.

In Fig. l, i denotes the magnet coil surrounded by the circular steel jacket 2. The pole pieces have the form of cones whose vertexes are pro- 35 vided with a central axially directed aperture. The apertured pole piece 3 facing the object has the form of an inverted obtuse-angled cone, whereas the angle of the apertured pole piece i is smaller, acute in the present case. The cross- 40 section of the pole pieces is chosen in such a manner that the magnetic saturation occurs at the ends of both pole pieces practically at the same time. To this end it is necessary to give the pole piece t a larger cross-section than the pole piece 3 in view of the leakage flux.

In the embodiment shown in Fig. 1 the concave sides of both pole pieces extend outwardly. However, the arrangement may be modified so 50 that the convex side of the pole piece 3 faces the object as shown schematically in Fig. 2. Also in this case the cross-section of the pole pieces may be so dimensioned that the saturation occurs at the same time at the ends of both 55 pole pieces.

What is claimed is:

1. A magnetic lens having an asymmetric pole system, and energizing means for developing a magnetic flux in said pole system, said system comprising an annular magnet core having in- 5 wardly extending cone-shaped pole shoes provided at its ends, the apexes of said cones being closely adjacent to one another and the cone angle of one pole shoe being relatively large and that of the other pole shoe being relatively small, 10 so that the apex portions of said cones both become located closely adjacent to one end of said annular core, the cross section of said pole shoes being dimensioned relatively to the flux density so that at least the ends of both pole shoes be- 15 come saturated at the same time and at the same flux density when the core is energized.

2. A magnetic lens having an asymmetric pole system, and energizing means for developing a magnetic flux in said pole system, said system 20 comprising an annular magnet core having inwardly extending cone-shaped pole shoes provided at its ends, the apexes of said cones being closely adjacent to one another and the cone angle of one pole shoe being relatively large and 25 that of the other pole shoe being relatively small, so that the apex portions of said cones both become located closely adjacent to one end of said annular core, the cross section of said pole shoes being dimensioned relatively to the flux density so that at least the ends of both pole shoes become saturated at the same time and at the same flux density when the core is energized, and a perforation in each pole shoe apex located in the common central axis of the shoes.

3. A magnetic lens having an asymmetric pole system and energizing means for developing a magnetic flux in said pole system, said system comprising an annular magnet core having inwardly extending cone-shaped pole shoes provided at its ends, the angles of said cones opposing one another and their apexes being closely adjacent to one another, the cone of one pole shoe including a relatively large obtuse angle and the cone of the other pole shoe including a relatively small acut angle, so that the apex portions of both cones become located closely adjacent to one end of said annular core, the cross section of said pole shoes being dimensioned relatively to the flux density so that at least the ends of both pole shoes become saturated at the same time and at the same flux density when the core is energized.

4. A magnetic lens having an asymmetric pole system, and energizing means for developing a pole shoe including a relatively large obtuse an-j gle, and the cone of the other pole shoe including a relatively small acute angle, so that the.

apex portions of both cones become located closely adjacent to one end of said annular core, the cross section of said pole shoes being dimensioned relatively to the flux density so that at least the ends, of both pole shoes become saturated at the same time and at the same flux density when the core is energized.

BODO VON BORRIES. ERNST RUSKA. 

